1. Technical Field
The present invention relates to a nonvolatile semiconductor memory circuit, and more particularly, to a nonvolatile semiconductor memory circuit for controlling a sensing voltage.
2. Related Art
In general, a unit memory cell of a phase change random access memory (PCRAM) includes one switching element which is connected to a word line, for example, one variable resistor (GST: germanium-antimony-tellurium) which is connected to a diode and a bit line. The PCRAM can store data in the memory cell by controlling the reversible phase change of the variable resistor (GST) using electrical pulses.
For example, if a pulse current is applied to the memory cell for a predetermined time, the state of the variable resistor (GST) can be established as an amorphous state (or a reset state). If the pulse current is applied to the memory cell for a time longer than the predetermined time, the state of the variable resistor (GST) can be established as a crystalline state (or a set state).
Such a PCRAM discriminates data by sensing an amount of current conforming to a change in resistance. For example, a current sense amplifier compares a sensing voltage, into which the amount of current is converted, with a reference voltage and discriminates data as “1” or “0”.
In general, the current sense amplifier permits current flow using a voltage higher than the voltage of an internal circuit, for example, a voltage of a VPP level. However, since another circuit uses a voltage of a CMOS level lower than the voltage of the VPP level, for example, a voltage of a VPERI level, the sensing voltage should be converted into a voltage for use in the cell. Under this situation, the output voltage of the current sense amplifier is level-shifted to a predetermined level by using a reference voltage, such that a signal having a voltage level for use in the cell can be provided. While a cross-coupled type sense amplifier is generally used as a level shifting circuit for this purpose, since the cross-coupled type sense amplifier has a substantial footprint due to its inherent characteristics, difficulties are likely to be caused in terms of disposal and layout thereof.